entropy.c

Go to the documentation of this file.

/*
 * Copyright (C) 2012 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
FILE_SECBOOT ( PERMITTED );

/** @file
 *
 * Entropy source
 *
 * This algorithm is designed to comply with ANS X9.82 Part 4 (April
 * 2011 Draft) Section 13.3.  This standard is unfortunately not
 * freely available.
 */

#include <stdint.h>
#include <assert.h>
#include <string.h>
#include <errno.h>
#include <ipxe/crypto.h>
#include <ipxe/hash_df.h>
#include <ipxe/entropy.h>

/* Disambiguate the various error causes */
#define EPIPE_REPETITION_COUNT_TEST \
        __einfo_error ( EINFO_EPIPE_REPETITION_COUNT_TEST )
#define EINFO_EPIPE_REPETITION_COUNT_TEST \
        __einfo_uniqify ( EINFO_EPIPE, 0x01, "Repetition count test failed" )
#define EPIPE_ADAPTIVE_PROPORTION_TEST \
        __einfo_error ( EINFO_EPIPE_ADAPTIVE_PROPORTION_TEST )
#define EINFO_EPIPE_ADAPTIVE_PROPORTION_TEST \
        __einfo_uniqify ( EINFO_EPIPE, 0x02, "Adaptive proportion test failed" )

/**
 * Initialise repetition count test
 *
 * @v source            Entropy source
 */
static void repetition_count_test_init ( struct entropy_source *source ) {
        struct entropy_repetition_count_test *test =
                &source->repetition_count_test;

        /* Sanity checks */
        assert ( test->repetition_count == 0 );
        assert ( test->cutoff > 0 );
}

/**
 * Perform repetition count test
 *
 * @v source            Entropy source
 * @v sample            Noise sample
 * @ret rc              Return status code
 *
 * This is the Repetition Count Test defined in ANS X9.82 Part 2
 * (October 2011 Draft) Section 8.5.2.1.2.
 */
static int repetition_count_test ( struct entropy_source *source,
                                   noise_sample_t sample ) {
        struct entropy_repetition_count_test *test =
                &source->repetition_count_test;

        /* A = the most recently seen sample value
         * B = the number of times that value A has been seen in a row
         * C = the cutoff value above which the repetition test should fail
         */

        /* 1.  For each new sample processed:
         *
         * (Note that the test for "repetition_count > 0" ensures that
         * the initial value of most_recent_sample is treated as being
         * undefined.)
         */
        if ( ( sample == test->most_recent_sample ) &&
             ( test->repetition_count > 0 ) ) {

                /* a) If the new sample = A, then B is incremented by one. */
                test->repetition_count++;

                /*    i.  If B >= C, then an error condition is raised
                 *        due to a failure of the test
                 */
                if ( test->repetition_count >= test->cutoff ) {
                        DBGC ( source, "ENTROPY %s excessively repeated "
                               "value %d (%d/%d)\n", source->name, sample,
                               test->repetition_count, test->cutoff );
                        return -EPIPE_REPETITION_COUNT_TEST;
                }

        } else {

                /* b) Else:
                 *    i.  A = new sample
                 */
                test->most_recent_sample = sample;

                /*    ii. B = 1 */
                test->repetition_count = 1;
        }

        return 0;
}

/**
 * Initialise adaptive proportion test
 *
 * @v source            Entropy source
 */
static void adaptive_proportion_test_init ( struct entropy_source *source ) {
        struct entropy_adaptive_proportion_test *test =
                &source->adaptive_proportion_test;

        /* Sanity checks */
        assert ( test->sample_count == 0 );
        assert ( test->repetition_count == 0 );
        assert ( test->cutoff > 0 );

        /* Ensure that a new test run starts immediately */
        test->sample_count = ADAPTIVE_PROPORTION_WINDOW_SIZE;
}

/**
 * Perform adaptive proportion test
 *
 * @v source            Entropy source
 * @v sample            Noise sample
 * @ret rc              Return status code
 *
 * This is the Adaptive Proportion Test for the Most Common Value
 * defined in ANS X9.82 Part 2 (October 2011 Draft) Section 8.5.2.1.3.
 */
static int adaptive_proportion_test ( struct entropy_source *source,
                                      noise_sample_t sample ) {
        struct entropy_adaptive_proportion_test *test =
                &source->adaptive_proportion_test;

        /* A = the sample value currently being counted
         * S = the number of samples examined in this run of the test so far
         * N = the total number of samples that must be observed in
         *     one run of the test, also known as the "window size" of
         *     the test
         * B = the current number of times that S (sic) has been seen
         *     in the W (sic) samples examined so far
         * C = the cutoff value above which the repetition test should fail
         * W = the probability of a false positive: 2^-30
         */

        /* 1.  The entropy source draws the current sample from the
         *     noise source.
         *
         * (Nothing to do; we already have the current sample.)
         */

        /* 2.  If S = N, then a new run of the test begins: */
        if ( test->sample_count == ADAPTIVE_PROPORTION_WINDOW_SIZE ) {

                /* a.  A = the current sample */
                test->current_counted_sample = sample;

                /* b.  S = 0 */
                test->sample_count = 0;

                /* c. B = 0 */
                test->repetition_count = 0;

        } else {

                /* Else: (the test is already running)
                 * a.  S = S + 1
                 */
                test->sample_count++;

                /* b.  If A = the current sample, then: */
                if ( sample == test->current_counted_sample ) {

                        /* i.   B = B + 1 */
                        test->repetition_count++;

                        /* ii.  If S (sic) > C then raise an error
                         *      condition, because the test has
                         *      detected a failure
                         */
                        if ( test->repetition_count > test->cutoff ) {
                                DBGC ( source, "ENTROPY %s excessively "
                                       "repeated value %d (%d/%d)\n",
                                       source->name, sample,
                                       test->repetition_count, test->cutoff );
                                return -EPIPE_ADAPTIVE_PROPORTION_TEST;
                        }
                }
        }

        return 0;
}

/**
 * Get entropy sample
 *
 * @v source            Entropy source
 * @ret entropy         Entropy sample
 * @ret rc              Return status code
 *
 * This is the GetEntropy function defined in ANS X9.82 Part 2
 * (October 2011 Draft) Section 6.5.1.
 */
static int get_entropy ( struct entropy_source *source,
                         entropy_sample_t *entropy ) {
        noise_sample_t noise;
        int rc;

        /* Any failure is permanent */
        if ( ( rc = source->rc ) != 0 )
                goto err_broken;

        /* Get noise sample */
        if ( ( rc = get_noise ( source, &noise ) ) != 0 )
                goto err_get_noise;

        /* Perform Repetition Count Test and Adaptive Proportion Test
         * as mandated by ANS X9.82 Part 2 (October 2011 Draft)
         * Section 8.5.2.1.1.
         */
        if ( ( rc = repetition_count_test ( source, noise ) ) != 0 )
                goto err_repetition_count_test;
        if ( ( rc = adaptive_proportion_test ( source, noise ) ) != 0 )
                goto err_adaptive_proportion_test;

        /* We do not use any optional conditioning component */
        *entropy = noise;

        return 0;

 err_adaptive_proportion_test:
 err_repetition_count_test:
 err_get_noise:
        source->rc = rc;
 err_broken:
        return rc;
}

/**
 * Initialise startup test
 *
 * @v source            Entropy source
 */
static void startup_test_init ( struct entropy_source *source ) {
        struct entropy_startup_test *test = &source->startup_test;

        /* Sanity check */
        assert ( test->tested == 0 );
        assert ( test->count > 0 );
}

/**
 * Perform startup test
 *
 * @v source            Entropy source
 * @ret rc              Return status code
 */
static int startup_test ( struct entropy_source *source ) {
        struct entropy_startup_test *test = &source->startup_test;
        entropy_sample_t sample;
        int rc;

        /* Perform mandatory number of startup tests */
        for ( ; test->tested < test->count ; test->tested++ ) {
                if ( ( rc = get_entropy ( source, &sample ) ) != 0 ) {
                        DBGC ( source, "ENTROPY %s failed: %s\n",
                               source->name, strerror ( rc ) );
                        return rc;
                }
        }

        return 0;
}

/**
 * Enable entropy gathering
 *
 * @v source            Entropy source
 * @ret rc              Return status code
 */
int entropy_enable ( struct entropy_source *source ) {
        int rc;

        /* Refuse to enable a previously failed source */
        if ( ( rc = source->rc ) != 0 )
                return rc;

        /* Enable entropy source */
        if ( ( rc = source->enable() ) != 0 ) {
                DBGC ( source, "ENTROPY %s could not enable: %s\n",
                       source->name, strerror ( rc ) );
                source->rc = rc;
                return rc;
        }

        /* Sanity check */
        assert ( source->min_entropy_per_sample > 0 );

        /* Initialise test state if this source has not previously been used */
        if ( source->startup_test.tested == 0 ) {
                repetition_count_test_init ( source );
                adaptive_proportion_test_init ( source );
                startup_test_init ( source );
        }

        DBGC ( source, "ENTROPY %s enabled\n", source->name );
        return 0;
}

/**
 * Enable and test entropy source
 *
 * @v source            Entropy source
 * @ret rc              Return status code
 */
static int entropy_enable_and_test ( struct entropy_source *source ) {
        int rc;

        /* Enable source */
        if ( ( rc = entropy_enable ( source ) ) != 0 )
                goto err_enable;

        /* Test source */
        if ( ( rc = startup_test ( source ) ) != 0 )
                goto err_test;

        DBGC ( source, "ENTROPY %s passed %d startup tests\n",
               source->name, source->startup_test.count );
        return 0;

 err_test:
        entropy_disable ( source );
 err_enable:
        assert ( source->rc == rc );
        return rc;
}

/**
 * Enable first working entropy source
 *
 * @v source            Entropy source to fill in
 * @ret rc              Return status code
 */
static int entropy_enable_working ( struct entropy_source **source ) {
        int rc;

        /* Find the first working source */
        rc = -ENOENT;
        for_each_table_entry ( *source, ENTROPY_SOURCES ) {
                if ( ( rc = entropy_enable_and_test ( *source ) ) == 0 )
                        return 0;
        }

        DBGC ( *source, "ENTROPY has no working sources: %s\n",
               strerror ( rc ) );
        return rc;
}

/**
 * Disable entropy gathering
 *
 * @v source            Entropy source
 */
void entropy_disable ( struct entropy_source *source ) {

        /* Disable entropy gathering, if applicable */
        if ( source->disable )
                source->disable();

        DBGC ( source, "ENTROPY %s disabled\n", source->name );
}

/**
 * Create next nonce value
 *
 * @ret nonce           Nonce
 *
 * This is the MakeNextNonce function defined in ANS X9.82 Part 4
 * (April 2011 Draft) Section 13.3.4.2.
 */
static uint32_t make_next_nonce ( void ) {
        static uint32_t nonce;

        /* The simplest implementation of a nonce uses a large counter */
        nonce++;

        return nonce;
}

/**
 * Obtain entropy input temporary buffer
 *
 * @v min_entropy       Min-entropy required
 * @v tmp               Temporary buffer
 * @v tmp_len           Length of temporary buffer
 * @ret rc              Return status code
 *
 * This is (part of) the implementation of the Get_entropy_input
 * function (using an entropy source as the source of entropy input
 * and condensing each entropy source output after each GetEntropy
 * call) as defined in ANS X9.82 Part 4 (April 2011 Draft) Section
 * 13.3.4.2.
 */
int get_entropy_input_tmp ( min_entropy_t min_entropy, uint8_t *tmp,
                            size_t tmp_len ) {
        struct entropy_source *source;
        struct {
                uint32_t nonce;
                entropy_sample_t sample;
        } __attribute__ (( packed )) data;;
        uint8_t df_buf[tmp_len];
        min_entropy_t entropy_total;
        unsigned int num_samples;
        unsigned int i;
        int rc;

        /* Enable entropy gathering */
        if ( ( rc = entropy_enable_working ( &source ) ) != 0 )
                goto err_enable_working;

        /* Sanity checks */
        assert ( source->startup_test.count > 0 );
        assert ( source->startup_test.tested >= source->startup_test.count );

        /* 3.  entropy_total = 0 */
        entropy_total = MIN_ENTROPY ( 0 );

        /* 4.  tmp = a fixed n-bit value, such as 0^n */
        memset ( tmp, 0, tmp_len );

        /* 5.  While ( entropy_total < min_entropy ) */
        for ( num_samples = 0 ; entropy_total < min_entropy ; num_samples++ ) {
                /* 5.1.  ( status, entropy_bitstring, assessed_entropy )
                 *       = GetEntropy()
                 * 5.2.  If status indicates an error, return ( status, Null )
                 */
                if ( ( rc = get_entropy ( source, &data.sample ) ) != 0 )
                        goto err_get_entropy;

                /* 5.3.  nonce = MakeNextNonce() */
                data.nonce = make_next_nonce();

                /* 5.4.  tmp = tmp XOR
                 *             df ( ( nonce || entropy_bitstring ), n )
                 */
                hash_df ( &entropy_hash_df_algorithm, &data, sizeof ( data ),
                          df_buf, sizeof ( df_buf ) );
                for ( i = 0 ; i < tmp_len ; i++ )
                        tmp[i] ^= df_buf[i];

                /* 5.5.  entropy_total = entropy_total + assessed_entropy */
                entropy_total += source->min_entropy_per_sample;
        }

        /* Disable entropy gathering */
        entropy_disable ( source );

        DBGC ( source, "ENTROPY %s gathered %d bits in %d samples\n",
               source->name, ( min_entropy / MIN_ENTROPY_SCALE ), num_samples );
        return 0;

 err_get_entropy:
        entropy_disable ( source );
        assert ( source->rc == rc );
 err_enable_working:
        return rc;
}

/* Drag in objects via entropy_enable */
REQUIRING_SYMBOL ( entropy_enable );

/* Drag in entropy configuration */
REQUIRE_OBJECT ( config_entropy );